To be able to use this bid strategy you need to have access to the optimisation on "Value" and to be whitelisted by a Facebook Representative to have the Return on Ad Spend (ROAS) bidding strategy.
With this bidding strategy you'll be able to define the lowest acceptable return on ad spend for your campaign. This bidding strategy might penalize the delivery though if the limit is set too high, as Facebook will try to respect the threshold defined for your ROAS.
This option will only be displayed when you optimize your campaign in value and when your campaign objective is conversions, catalog sales or app install.
More information about this bidding strategy can be found in the links below: